Fabrication of blazed gratings for X-ray spectroscopy using substrate conformal imprint lithography
Abstract
The majority of spectral lines relevant in high energy astrophysics exist at soft X-ray energies, where gratings dominate over microcalorimeters. Next-generation reflection gratings have been identified as a key technology to improve the spectroscopic capabilities of future X-ray observatories. Currently, the grating fabrication process centers on the production of a large-area (72 cm2) master template through techniques in electron-beam lithography, plasma etching and anisotropic wet etching in single-crystal Si. Then, many replicas are produced to populate a grating array, which intercepts and disperses the radiation coming to a focus in a Wolter-I telescope. Of importance is implenting a replication procedure that preserves the fidelity of the master grating template at a low cost. Traditionally, the Si master template has been used to stamp directly into a UV-curable resist coated on a fused silica substrate through the process of nanoimprint lithography (UV-NIL). Though the high stiffness of Si allows the desired inverse of the original pattern to be imprinted with high resolution, difficulties arise especially when imprinting over a large area. Substrate conformal imprint lithography (SCIL) is a relatively new commercial process intended to evade these problems. In contrast to UV-NIL, the SCIL process uses a flexible stamp formed from the master template for imprinting. The flexible stamp carries the inverse of the original pattern in a modified silicone rubber, which has increased stiffness compared to standard silicone used in soft lithography processes. This enables the positive of original pattern to be imprinted sequentially with high resolution while confroming to the bow of the substrate and reducing damage due to particulate contaminants. The desired inverse of the original pattern can be imprinted with SCIL by forming a second flexible stamp from the initial flexible stamp. Further, SCIL is compatible with an inorganic imprint resist that has been found to exhibit sub-nm RMS roughness when coated with Cr/Au for reflectivity. Here, data gathered from the Advanced Light Source characterizing the efficiency of the original Si grating and the non-inverted and inverted SCIL replicas are presented.
- Publication:
-
AAS/High Energy Astrophysics Division #16
- Pub Date:
- August 2017
- Bibcode:
- 2017HEAD...1610301M